A major source of many metals, particularly copper and gold, is sulfidic ores. In sulfidic ores, the metals are either present as or immobilized by stable metal sulfides, which are frequently nonreactive or slow reacting with lixiviants such as cyanide, ferric ion or sulfuric acid. To promote the dissolution of the metals in a lixiviant, the elements compounded with the metal (e.g., sulfide sulfur) are first be oxidized. In one approach, oxidation of the sulfide sulfur is induced by organisms, such as Thiobacillus Ferrooxidans and Thiobacillus Thiooxidans (commonly referred to as biooxidation or bioleaching).
Although biooxidation can be performed in a continuous stirred tank reactor, a common technique is to perform biooxidation in a heap. Compared to biooxidation in a continuous stirred reactor, heap biooxidation generally has lower capital and operating costs but a longer residence time and lower overall oxidation rate for the sulfide sulfur in the feed material.
In designing a heap biooxidation process, there are a number of considerations. First, it is desirable to have a relatively high heap permeability and porosity. Fine material can decrease heap permeability and porosity and result in channeling. Channeling can cause a portion of the material in the heap to have a reduced contact with the lixiviant, thereby limiting the degree of biooxidation of the material. Second, it is desirable that the residence time of the feed material in the heap (i.e., the time required for an acceptable degree of biooxidation) be as low as possible. Existing heap leaching processes typically have residence times of the heap on the pad of 12 months or more for an acceptable degree of biooxidation to occur.